1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for the production the same. More particularly, the present invention relates to a polymer dispersed liquid crystal display device utilizing a liquid crystal-polymer composite film in which liquid crystal droplets are partitioned by a matrix composed of polymer material.
2. Description of the Related Art
Liquid crystal display devices are used in a broad range of products such as projection-type display devices, direct-view type display devices, display panels, and windows with shutter functions. There are a variety of display modes applicable to liquid crystal display devices, as is described below:
There have been realized display devices of a TN (Twisted Nematic) mode or an STN (Super Twisted Nematic) mode, as display devices utilizing electrooptical effects. Both display modes use nematic liquid crystal. A display mode using FLC (Ferroelectric Liquid Crystal) has also been proposed. However, there display modes, since they require a polarizing plates, have the problem of low utilization efficiency of light. Moreover, these display modes require an aligning film and an orientation treatment in order to align the liquid crystal molecules.
Display modes which do not require any polarizing plates are, for example, a DS (Dynamic Scattering) mode and a PC (Phase Change) mode. However, these display modes have the problem of low contrast.
There have been proposed polymer dispersed liquid crystal display devices as display devices which require neither a polarizing plate nor an orientation treatment of the liquid crystal. The configuration of a polymer dispersed display device is such that a liquid crystal-polymer composite film in which liquid crystal droplets are partitioned by a matrix composed of polymer material (hereinafter, such a liquid crystal-polymer composite film is referred to as a `polymer dispersed liquid crystal`) is interposed between two opposing substrates.
Operating principles for polymer dispersed liquid crystal display devices are briefly described below:
When no voltage is applied, liquid crystal molecules within each liquid crystal droplet dispersed in the polymer matrix are in a random orientation state. Since liquid crystal molecules have birefringence, there arises intense scattering of light due to ununiformity of refractive indices within the liquid crystal droplets and difference between the refractive index of the polymer matrix and that of the liquid crystal. Because of the light scattering, the polymer dispersed liquid crystal display device does not transmit any light when voltage is not applied. That is, the display device is in an opaque state.
On the other hand, when a voltage is applied, liquid crystal molecules having positive dielectric anisotropy are oriented in such a way that the longitudinal axes thereof are in parallel to an applied electric field. Therefore, by using materials such that an ordinary refractive index of the liquid crystal molecules having positive dielectric anisotropy is identical with the refractive index of the polymer matrix, it becomes possible to obtain a light scattering state when no voltage is applied, and a transparent state when a voltage is applied. In other words, a polymer dispersed liquid crystal display device conducts display by controlling the amount of light to be transmitted rather than electrically controlling the intensity of light scattering.
The followings are examples of conventional methods for fabricating the above-mentioned polymer dispersed liquid crystal display devices:
(1) A method disclosed in Japanese National Publication No. 58-501631: Polymer capsules containing liquid crystal droplets are formed in the interspace between two opposing substrates. This method is a so-called polymerization induced phase separation method.
(2) A method disclosed in Japanese National Publication No. 61-502128: A PDLC (Polymer Dispersed Liquid Crystal) precursor as a mixture of liquid crystal material, a photocurable polymer, and a thermosetting polymer is sealed between a pair of opposing substrates, whereby liquid crystal droplets partitioned by a polymer matrix are formed.
(3) A method disclosed in Japanese Laid-Open Patent Publication No. 59-226322: A solution (PDLC precursor solution) of polymer material and liquid crystal material in a solvent which can dissolve both materials is applied on one of a pair of substrates and is dried so as to remove the solvent. Thereafter, the other substrate is applied to the said substrate.
However, a polymer dispersed liquid crystal display device fabricated by any of the above-mentioned conventional methods has the problem that unreacted polymer material may dissolve into the liquid crystal and/or that liquid crystal may dissolve into the polymer matrix. Therefore, the resultant polymer dispersed liquid crystal display device has the problem of high threshold voltage (as one of various electrooptical characteristics of the display device), thus deteriorating the display characteristics of the display device. Moreover, there is another problem that heating such a polymer dispersed liquid crystal display device would further promote such mutual elution, or mingling, of the polymer matrix and the liquid crystal phase. As a result, the electrooptical properties, such as threshold voltage, of the display device would deteriorate.
As for a guest-host type polymer dispersed liquid crystal display device fabricated according to a conventional method, there is a problem that dye molecules dissolve into the polymer matrix, thus lowering the contrast of the display device.
The above-mentioned problems are most prominant in cases where a polymer dispersed liquid crystal display device fabricated according to a conventional method is applied to a projection-type liquid crystal display device or a transmissive liquid crystal display device provided with a backlight. In these display devices, heating effects due to emission of light by a light source such as a metal halide lamp, a cold-cathode tube, an EL (Electroluminiscent) film, and the like prompt the above-mentioned problem of mutual elution, that is, elution of unreacted polymer material into the liquid crystal and elution of the liquid crystal into the polymer matrix. As a result, the display characteristics of the polymer dispersed liquid crystal display device are greatly deteriorated.
In cases where a polymer dispersed liquid crystal display device is fabricated by a photopolymerization induced phase separation method in which UV-rays (Ultra Violet rays) are used, a problem arises that the UV-rays used in the fabrication of the display device deteriorate the liquid crystal molecules, resulting in a polymer dispersed liquid crystal display device with poor electrooptical properties.
Moreover, according to a conventional method for fabricating a polymer dispersed liquid crystal display device, it is difficult to form the liquid crystal droplets in uniform size and shape. As a result, a conventional polymer dispersed liquid crystal display device has a problem that the steepness of electrooptical characteristics (voltage-light transmittance characteristics) is inadequate.